Treatment of base metal smelter slag

ABSTRACT

A process for treating base metal smelter slag containing from about 20 to about 60 percent by weight iron and at least one further metal selected from the group consisting of nickel, copper and cobalt comprises mixing base metal smelter slag with calcium sulphate and carbon and melting the resultant mixture to produce a metallic sulphide matte containing substantially all the further metal or metals and a secondary slag substantially free of the further metal or metals suitable for use as a cement.

RELATED APPLICATION

This invention claims priority from U.S. Provisional Patent Application No. 60/789,879 filed Apr. 7, 2006.

FIELD OF INVENTION

This invention relates to base metal smelter slag.

BACKGROUND OF THE INVENTION

It is well known that base metal smelter slag usually contains appreciable amounts of nickel, copper and cobalt, and various processes for recovering such metals have been proposed in the past. However, such previously proposed processes have not proved to be particularly successful.

Prior art of general interest is described in U.S. Pat. No. 5,593,495 issued Jan. 14, 1997; U.S. Pat. No. 5,626,646 issued May 6, 1997; U.S. Pat. No. 5,749,962 issued May 12, 1998; U.S. Pat. No. 5,865,872 issued Feb. 2, 1999; U.S. Pat. No. 5,968,258 issued Oct. 19, 1999; and U.S. Pat. No. 6,033,467 issued Mar. 7, 2000. The contents of these prior patents are hereby incorporated herein by reference.

SUMMARY OF THE INVENTION

It has now been discovered that a metallic sulphide matte which has a relatively low melting point and contains substantially all the nickel, copper and cobalt in the original base metal smelter slag can be obtained by mixing the original slag with optimum amounts of calcium sulphate and carbon and melting the mixture. Besides the sulphide matte, a secondary slag suitable for use as a cement is also produced.

The base metal smelter slag may be mixed with from at least 10 to about 60 percent by weight calcium sulphate and from about 2 to about 10 percent by weight carbon relative to the weight of the base metal smelter slag.

The process described above can be optimized such that a minimum amount of sulphide matte with relatively high concentrations of nickel, copper and cobalt and a maximum amount of secondary slag can be obtained.

Base metal smelter slag typically contains from about 20 to about 60 percent by weight iron in one or more of the following compounds together with amounts of nickel, copper and cobalt:

FeO.SiO₂

Fe₂O₃.SiO₂

Fe₃O₄.SiO₂

Other compounds such as CaO, Al₂O₃, MgO, S and TiO₂ may also be present.

Adding calcium sulphate and carbon and heating the resultant mixture in accordance with the invention produces reactions of the follow kind:

CaSO₄+2C→CaS+2CO₂↑

FeO+CaS→FeS+CaO

FeO.SiO₂+CaS→FeS+CaO.SiO₂

5(FeO.SiO₂)+CaSO₄+2C→CaO.₄FeO.5SiO₂+FeS+2CO₂↑

It will be understood that these equations are simplified versions of reactions which occur in practice.

The FeS matte contains substantially all the nickel, copper and cobalt which was in the original base metal smelter slag and the CaO.SiO₂ or CaO.₄Fe0.5SiO₂ is a secondary slag which is substantially free of nickel, copper and cobalt and can be used as a cement.

DESCRIPTION OF PREFERRED EMBODIMENTS

Examples of the present invention will now be described.

Example 1

A sample of base metal smelter slag was obtained from a slag heap that has been accumulated over about 50 years by a large nickel producer located in Sudbury, Ontario, Canada.

100 g of the sample were crushed to 1 mm (100% passing 18 mesh sieve) and mixed with 41 g of anhydrous calcium sulphate (100% passing 20 mesh sieve) and 7 g of carbon (100% passing 20 mesh sieve). The mixture was put in a clay crucible and the crucible was placed inside a propane fired drum furnace. The furnace was then heated for one hour to about 1300° C. Within three minutes, the carbon reacted with the anhydrous calcium sulphate to form calcium sulphide, which in turn reacted with the iron oxide contained in the slag sample. The reaction lasted a few minutes. The resultant molten material was poured into a cast iron mold to cause fast cooling of the molten metal and separation into two phases, namely a metallic sulphide matte and a secondary slag. The secondary slag was identified as wollastonite type slag. Samples of the original base metal smelter slag, metallic sulphide matte and the secondary slag were analysed.

The results were as follows (with the original base metal smelter slag being identified as slag and the secondary slag being identified as new slag):

Slag CaSO₄ C Crucible Total Total Iron Sulfide New Slag CO₂ Before 100 41 7 148 After 148 26.5 95 26.5 100%

MASS BALANCE Output (%) Mass Out (g) Distribution (%) Input Mass Iron New Iron New Iron New Slag CaSO4 C In (g) Sulfide Slag CO₂ Sulfide Slag CO₂ Sulfide Slag CO₂ Weight (g) 100 41 7 148 17.91 63.51 17.91 26.50 95.00 26.50 % % Ni 0.40 0.40 1.38 n.d. 0.37 92.50 Cu 0.54 0.54 1.95 n.d. 0.52 96.30 Co 0.20 0.20 0.75 n.d. 0.20 100.00 S 1.4 23.53 11.05 23.85 1.52 6.32 1.44 65.49 14.92 Fe 59.07 15.65 38.39 FeO 53.00 53.00 39.88 37.89 71.49 Si 0.41 0.11 SiO₂ 32.80 32.80 32.80 31.16 95.00 Al Al₂O₃ 9.72 9.72 9.58 9.10 93.62 Ca 0.37 0.10 CaO 2.10 41.18 18.98 13.82 13.13 69.18 Mg MgO 2.30 2.3 1.05 1.00 43.48

Example 2

In Example 1, the stoichiometric ratio of anhydrous calcium sulphate to the base metal smelter slag was 1:1.94 (corresponding to 52 g CaSO₄ and 100 g FeO.SiO₂). By adding less calcium sulphate, all the heavy metals from the original base metal smelter slag were removed as well. Accordingly, in this example, which is otherwise similar to Example 1, the minimum amounts of calcium sulphate and carbon needed for the reaction to take place efficiently were added, as follows:

Slag CaSO₄ C Crucible Total Total Iron Sulfide New Slag CO₂ Before 100 20 3 123 After 123 13 97 13 100%

MASS BALANCE Output (%) Mass Out (g) Distribution (%) Input Iron New Iron New Iron New Slag CaSO4 C Mass In (g) Sulfide Slag C Sulfide Slag CO₂ Sulfide Slag CO₂ Weight (g) 100 20 3 123 10.6 78.9 10.6 13 97 13 % % Ni 0.40 0.40 2.38 n.d. 0.31 77.50 Cu 0.54 0.54 3.62 n.d. 0.47 87.04 Co 0.20 0.20 1.38 n.d. 0.18 90.00 S 1.4 23.53 6.11 36.23 1.54 4.71 1.49 77.09 24.39 Fe 89.08 11.58 28.11 FeO 53.00 53.00 40.24 39.03 73.64 Si 0.54 0.07 SiO₂ 32.80 32.80 33.10 32.11 97.90 Al Al₂O₃ 9.72 9.72 9.38 9.10 93.62 Ca 0.77 0.10 CaO 2.10 41.18 10.34 13.54 13.13 69.18 Mg MgO 2.30 2.3 1.03 1.00 43.48

Example 3

The secondary slag from Example 1 was ground to 4000 Blaine and then blended with Type 1 normal Portland cement in different proportions to make 2″ cubes in accordance with the standards of the American Society for Testing Materials (ASTM). Two sets of ASTM 2″×2″ cubes were made, one with 25% by weight replacement and the other with 50% by weight replacement of the normal Portland cement portion by the secondary slag, and their compressive strengths were tested at 3,7 and 28 days and compared with a control using 100% Type 1 normal Portland cement. This test measures the compressive strength of the paste (cementitious fraction), and it is the very first test which needs to be done to see if a material has the potential of being a good supplementary cementing material.

The two ratio replacements, 25% and 50%, were chosen for the following reasons:

The 25% replacement is the amount that generally, in most concrete/ready mix applications, is replaced by a supplementary cementing material and also it is the replacement to measure the Pozzolanic Activity Index (P.A.I.) at 28 days (that is the minimum ASTM compressive strength for a 25% pozzolan replacement as a percentage of the control with 100% normal Portland cement: Min. P.A.I.>75%). The 50% replacement is the amount required for the ASTM Slag Activity Index (S.A.I.) at 28 days (that is the minimum ASTM compressive strength for a 50% slag replacement as a percentage of the control with 100% normal Portland cement: min. S.A.I.>80%).

The compressive strength was measured using a certified compressive strength machine known as Forney F-250E-DR2000.

The following are the results of the compressive strength tests:

Compressive Strength Test Mix Proportion Compressive in gm Strength in psi Cement Sand New Slag Water 3 day 7 day 28 day Control 500 1375 — 242.5 4336 4562 6230 #1 375 1375 125 242.5 3760 4588 6369 #2 250 1375 250 242.5 2262 3625 6395

The Pozzolanic Activity Index (P.A.I.) at 28 days has to be at least 75% of a control sample. The P.S.A.I. with Portland Cement according to the ASTM Standards is calculated in the following manner:

Pozzolanic Strength Activity Index with Portland cement=(A/B)×100

Where: A=average compressive strength of test mix cubes made with 75% Portland cement and 25% by mass of the tailing cement (6369 psi)

-   -   B=average compressive strength of control mix cubes (6280 psi)

For the New Secondary Slag the P.A.I.=6369/6280×100=101.4%.

The Slag Activity Index (S.A.I.) at 28 days has to be at least 80% of a control sample. The S.A.I. with Portland cement according to the ASTM Standards is calculated in the following manner:

Slag Activity Index with Portland cement=(A/B)×100

Where: A=average compressive strength of text mix cubes made with 50% Portland cement and 50% by mass of the tailing cement (6395 psi)

-   -   B=average compressive strength of control mix cubes (6280 psi)

From these results, it is clear that a very good supplementary cementing material can be produced from the new secondary slag from Example 1.

Example 4

New secondary slag from Example 2 was ground to 4000 Blaine and then blended with Type 1 normal Portland cement in different proportions to make 2″ cubes in accordance with the standards of the American Society for Testing Materials (ASTM) as in Example 3.

The following are the results of the compressive strength tests:

Compressive Strength Test Mix Proportion Compressive in gm Strength in psi Cement Sand New Slag Water 3 day 7 day 28 day Control 500 1375 — 242.5 3696 5220 6511 #1 375 1375 125 242.5 3314 5061 6569 #2 250 1375 250 242.5 2340 3715 6471

The advantages of the present invention are clearly evident.

Other embodiments and examples of the invention will now be readily apparent to a person skilled in the art, the scope of the invention being defined in the appended claims. 

1. A process for treating base metal smelter slag containing from about 20 to about 60 percent by weight iron and at least one further metal selected from the group consisting of nickel, copper and cobalt, the process comprising mixing base metal smelter slag with calcium sulphate and carbon and melting the resultant mixture to produce a metallic sulphide matte containing substantially all said further metal or metals and a secondary slag substantially free of said further metal or metals suitable for use as a cement.
 2. A process according to claim 1 wherein the base metal smelter slag is mixed with from about 10 to about 60 percent by weight calcium sulphate and from about 2 to about 10 percent by weight carbon relative to the weight of the base metal smelter slag. 